Plants can take up CWD-causing prions from soil in the lab. What happens if they are eaten?

Johan Neven / Flickr

When Christopher Johnson, PhD, set out to study whether lab mice fed prion-contaminated plants developed neurodegenerative disease, he expected the plants to take up only small prion clusters, but they absorbed large clusters characteristic of prion diseases in deer and other animals.

Then again, "Prions are constantly surprising," Johnson, a study coauthor and deputy director of the Office of Science Quality and Integrity at the US Geological Survey in Reston, Virginia, told CIDRAP News. "But perhaps we shouldn't ever be allowed to be surprised by them, because they are so resistant to degradation, and they are so resilient that finding prions in unusual settings is maybe something that we should all begin to just expect."

Prions are infectious misfolded proteins that cause fatal neurodegenerative diseases such as chronic wasting disease (CWD) in cervids like deer and elk, scrapie in sheep and goats, bovine spongiform encephalopathy (BSE, or "mad cow" disease) in cattle, and Creutzfeldt-Jakob disease in humans.

In the case of CWD, once an animal is infected, it can spread the disease through direct contact, saliva, antler velvet, urine, feces, and carcasses, and the prions can persist in the environment for years. Once an animal is exposed, the incubation period in a host—the time before symptoms appear—is thought to be up to 2 years.

But given the rapid spread of CWD throughout North America and parts of Europe and Asia, scientists question whether it is also being transmitted through a different route, such as the ingestion of contaminated plants.

Prions remain infectious in plants

While researchers have been experimenting with protein uptake into plants since the 1970s, Johnson and colleagues' laboratory study, published in iScience in December, takes those investigations a step further. They demonstrated that alfalfa, barley, and Arabidopsis thaliana, a small plant from the mustard family called thale cress and other names, all accumulated sufficient prions from contaminated soil in their above-ground tissues to cause mice that ingested the plant tissues to develop prion disease.

"There was a previous study that looked to see if plants could become surface-contaminated or potentially accumulate prions at all, and it looked like they could," Johnson said. "Our study shows that, under these lab conditions, consumption of contaminated plants could cause an infection," with implications for wildlife conservation, agriculture, and public health.

The lead author of the previous study, Sandra Pritzkow, PhD, associate professor of neurology at McGovern Medical School at UTHealth Houston, said that prions can't replicate, or reproduce, in plants, so plants, like earthworms, should be considered potential carriers rather than hosts.

Published in Cell Reports in 2015, Pritzkow's lab study showed that prions diluted in brain, urine, or feces can bind to and be taken up by wheat-grass roots and leaves. They also showed that wild-type hamsters that ate contaminated grass incubated in infected brain material were subsequently infected, which the authors said suggests "a possible role of environmental prion contamination in the horizontal transmission of the disease."

Two deer eating plants — Dennis Church / Flickr cc

Finding that plants can accumulate prions from contaminated soil to biologically relevant levels in the above-ground tissues—at least in the lab—is concerning, Johnson said: "Plants are obviously moved in the case of agriculture across the continents, the world, and so you know that brings up the concern that these plants may be able to take prions to new places or expose animals or humans when they're in a place where perhaps chronic wasting disease isn't thought to occur."

Stuart Lichtenberg, PhD, director of environmental chemistry at the Minnesota Center for Prion Research and Outreach (MNPRO) and a working group member on CIDRAP's CWD contingency-planning project at the University of Minnesota, pointed out that, given prion behavior, the likelihood of eliminating prion infectively in crops is low.

If cultivated crops were to take up sufficient amounts of prions to be infectious, "Those prions would remain infectious for as long as that foodstuff or feed or what have you was stored," he said. "If a farmer was growing a field of alfalfa and used that as a forage for his animals, stored for a season or two, even after cutting that grass, alfalfa, hay, it's still going to be infected with prions when it's fed to animals."

And remediation of prion uptake in crops is a relatively unexplored area. High concentrations of chemicals such as bleach, for example, can work if you can zero in on a hot spot, Lichtenberg said, "but obviously there are issues with spraying a whole lot of bleach on, say, a cornfield."

Norway moves to prevent import of contaminated plants

The possibility of importing contaminated plants prompted Norway in 2018 to ban the import of hay and straw for animal feed from CWD-positive areas outside the European Economic Area.

The regulation specifies that hay and straw must be accompanied by manufacturer confirmation that the product has been stored for at least 2 months and that it wasn't harvested from a farm that used animal manure fertilizer in the past 2 years, as well as a regulatory veterinarian attestation that the product doesn't come from a CWD-positive area.

Kristin Ruud Alvseike, a veterinarian at the Norwegian Food Safety Authority, said the main concern behind the ban was the spread of CWD from North America to cervids in Norway rather than to non-cervid animals and humans.

"Our goal is to confine the disease, or, if possible, eradicate it," she said. "The regulation of hay and straw is one of many elements we have implemented in this matter."

She added that the epidemiologic situation in Norway is very different from the one in the United States. "Our first case was detected in 2016; since then we have detected 21 cases of contagious CWD, all of them in wild reindeer," she said. "The last case was in 2022. This indicates that we are in an early phase of the disease."

No evidence that prions in plants infect deer

But while it may be possible for deer or non-cervid animals such as people to be exposed to prions in contaminated plants, there has been no evidence of infection, and the CWD-prion species barrier, which prevents zoonotic transmission, is thought to be high in humans.

[CWD] can still change. Also, it's spreading a lot, and we don't know why. It's very difficult to control. And therefore, I think it's more dangerous.

Sandra Pritzkow, PhD

"There is the possibility that other hosts can get infected with a chronic wasting disease with a higher zoonotic potential, and then a new disease can develop," Pritzkow said. "This is a real possibility, but it is yet unclear how concerned we should be with this potential problem."

Yet she said that she considers CWD the most dangerous prion disease because it is relatively new, not much is known about it, and most deer live in uncontrolled environments. This is in contrast to scrapie, which has existed for hundreds of years without a known case of human infection—perhaps because of a species barrier and because most sheep and goats are farmed animals kept in more contained environments.

"[CWD] can still change," Pritzkow said. "Also, it's spreading a lot, and we don't know why. It's very difficult to control. And therefore, I think it's more dangerous."

Lichtenberg also remains curious about how CWD is spreading so quickly in the wild, given that deer don't eat other deer and that they are fairly well spaced in the environment. "It gets at the idea that there could be both direct animal-to-animal transmission and a form of indirect transmission taking place," he said.

Two reindeer — David Stanley / Flickr cc

While rodents have been experimentally infected with prions via ingestion of plants, it has never been shown that animals such as deer can be infected this way. "There's some pretty substantial differences between the digestive physiology of mice versus, say deer, and that could radically affect disease transmission from plants," Lichtenberg said.

Yet possible species spillover is worthy of caution and further investigation. "It takes very, very small [prion] doses to sicken vulnerable deer," he said. "Right now, I would say we can be cautiously optimistic that it's not a likely scenario to happen, but once again, that doesn't mean it couldn't happen," he said.

Johnson said that experts have been concerned about scavengers and other predators acquiring CWD from infected deer. "And plants that could be contaminated with prions are consumed by all sorts of varieties of animals and so could represent a new pathway that other non-target animals could acquire disease," he said.

Plants could be a more important mode of CWD exposure than the soil, which grazing animals also eat. "But they eat a lot more plants, so if those plants are contaminated, that's a much more direct route by which they would be exposing themselves to potentially much more infectious agents," Johnson said.

Many unknowns, research avenues

Jason Bartz, professor and chair of the Department of Medical Microbiology and Immunology at Creighton University, said that different CWD strains can bind to the soil with different affinity and that the weathering of prions is both soil- and strain-dependent.

"How does this change or influence the biology of CWD strain distribution?" said Bartz, who is also a cochair of a CIDRAP CWD contingency-planning work group. "We know that different strains of prions can have different pathogeneses, tropisms, and zoonotic potentials, so all these have some pretty big implications for how we manage the disease."

But so much is yet unknown, including the number of existing CWD strains, he said, noting that there is no universal definition of a strain or what makes one strain different from another.

Lichtenberg said that research on plants' prion uptake is still nascent. "It's technically challenging because it involves mixing a number of different fields together," he said. "You need experts in plant physiology, animal rearing, and general prion work, and it's somewhat rare."

Future research, Johnson said, should examine prion uptake in the ruminant digestive system, whether through use of a simulated system or in ruminants in a secure agricultural setting. Other avenues of interest include the kinetics of uptake, how long prions persist in plants, and the accumulation of prions in pollen and fruit.